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You must mean that all states the original once held slaves.
Many states outlawed slavery and it was outlawed in other new states by the Northwest Ordinance
Increasing numbers of settlers and land speculators were attracted to what are now the states of Ohio, Indiana, Illinois, Michigan and Wisconsin. This pressure together with the demand from the Ohio Land Company, soon to obtain vast holdings in the Northwest, prompted the Congress to pass this Ordinance. The area opened up by the Ordinance was based on lines originally laid out in 1784 by Thomas Jefferson in his Report of Government for Western Lands. The Ordinance provided for the creation of not less than three nor more than five states. In addition, it contained provisions for the advancement of education, the maintenance of civil liberties and the exclusion of slavery. http://www.earlyamerica.com/earlyamerica/milestones/ordinance/index.html